I already brought up a guy with a 343 chapter/issue manga in Kentaro Miura (Berserk), and Takehiko Inoue, a man who wrote, and drew, a 276 issue manga (Slam Dunk, from 1990-1996), and currently Vagabond (a manga based off of Musashi Miyamoto's dairy/journal), which has 327 chapters/issues currently, while he's doing a 80+ chapter manga (REAL). These are living legends in the industry, too.
Google their names and be mystified by their artwork!!!
To be fair--most manga artists do NOT work solo. Most have a lot of inkers and background artists. Some have been known to have any many as 20-30 guys pitching in and helping out. So, while one person is putting their name on it--it's not really the same animal as Stan Sakai sitting down and pencilling and inking 200+ issues of Usagi Yojimbo.
In terms of pure drawing--aren't there artists at Archie who drew 500-700 issues?
Well that's because, unlike in the west, a lot of manga come out weekly. They need to do that. These guys get like 3-4 hours of sleep, and they're breaks are usually like 1-2 weeks.
However, these two artists I picked are monthly guys; besides at times using some inkers when they're sick, they do it on their own. When they take breaks, the manga is NOT continued by anyone else.
Kentaro Miura:
Takehiko Inoue (via paint brush, as he doesn't draw his manga with pencil, or pen, for...the challenge, I guess lol, who knows):
As much as I love western comics, you'll only get this kind of detail from the pages of (monthly) mangas.
Studio Gaga assist with the art in Berserk though, doing backgrounds and the like. Thats a team of at least 5 guys.